Methods for suppressing stimulated Brillouin scattering (SBS) in a conventional art will be described as follows.
In a SBS suppression method, a separate phase modulator is used for generating a frequency chirp of signal, thereby extending a width of spectrum.
However, in the conventional method using such a phase modulator, there is a disadvantage that besides an intensity modulator used for modulating signal, an additional phase modulator must be used.
In a second SBS suppression method, in a wavelength division multiplexing (WDM) link, a width of spectrum is extended by a phase modulation through a cross phase modulation (XPM) between signal channels.
An example of the second SBS suppression method is described in a paper, Y. Horiuchi et al., “SBS suppression effects induced by XPM in high power WDM repeaterless transmission,” Electronics Letters, vol.34, no.4, p390–391, 1998.
In the above example, the SBS generated in a wavelength division multiplexing (WDM) link of 2.5 Gb/s×8 channels is suppressed by using a cross phase modulation (XPM). As a result, although a power of +18 dBm per a channel is inputted, the SBS is not generated, thereby transmitting a signal through pure silica core fibers (SMF) of 423 km without using a repeater.
However, in the wavelength division multiplexing (WDM) link, the use of the conventional method using a phase modulation effect by cross phase modulation between the signal channels is limited, because conditions generating a phase modulation effect are determined by a transmission speed and a light intensity between each signal channel within the wavelength division multiplexing WDM channel, in a case that the transmission speed and a light intensity are changed, the phase modulation effect is also changed.
In a third method, a low frequency and a modulated amplitude are applied to a semiconductor laser, thereby extending a width of a spectrum.
The above example is described in U.S. Pat. No. 5,329,396 filed by D. A. Fishman, entitled as “Reduction of stimulated Brillouin scattering in a fiber having reduced Brillouin scattering”, and issued on Jul. 12, 1994.
In such an example, as a method that a critical value of stimulated Brillouin scattering (SBS) being generated in an optical fiber is raised up, a waveform of a low frequency is applied to a general laser diode (LD) and modulated it and so a valid line width of the laser diode (LD) is extended, thereby raising up the critical intensity of stimulated Brillouin scattering.
However, although the conventional method for applying a low frequency and a modulated amplitude to the semiconductor laser is widely used, in a case that a direct modulation can not apply, like an optical fiber laser, it is impossible to adapt.
In a fourth method, a duobinary method is used as a modulation method.
The above example is described in U.S. Pat. No. 5,917,638 filed by F. Thorkild et al., entitled as “Duo-binary signal encoding”, and issued on Jun. 29, 1999.
In the above example, in order to raise up the threshold of stimulated Brillouin scattering (SBS) phenomenon, a duobinary modulation method is used as a modulation method of a transmission light, thereby raising up the threshold of stimulated Brillouin scatting SBS. By using this method, when comparing it to the conventional binary modulation method, the threshold of stimulated Brillouin scattering (SBS) is increased by 12.6 dB at a transmission speed of 10 Gb/s and at this time, power penalty is less than 1 dB.
However, in the conventional method using the duobinary modulation method, a new method, unlike the conventional modulation method, must be used, the implementation method is complicate and its technology is not yet completed until now.
In a fifth method, there is a method that an optical fiber in which stimulated Brillouin scattering dose not well generated is used.
The above example is described in U.S. Pat. No. 5,851,259 filed by J. B. Clayton et al., entitled as “Method for making Ge-doped optical fibers having reduced Brillouin scattering”, and issued on Dec. 22, 1998.
In the above example, when making an optical fiber being added germanium Ge, in case that a properly modulated tension is applied, stimulated brillouin scattering can be suppressed without a loss of the optical fiber and a large change of a value of dispersion. At this time, the used tension is modulated by 10–50 g in a low case and 150–200 g in a high case.
However, in the conventional method using an optical fiber in which stimulated brillouin scattering SBS does not generated in well, there are several disadvantages that a conventional optical fiber can not be used and all optical fibers must be embedded in new.